1. Field of the Invention
The present invention relates to an image forming optical system suitable for imaging apparatus using solid-state image sensing devices, such as CCD (Charge Coupled Device) and CMOS (Complementary Metal-Oxide Semiconductor), etc.
2. Description of the Related Art
In recent years, motorcar equipped with a camera for vehicle that is mounted on vehicle has been widely used. The camera for vehicle was used only for observation by eyes in a monitor at the beginning stage. However, recently, it has been increasingly used also as camera for much advanced sensor, for detection of a white line, detection of deviation from traffic lane, detection of an obstacle, detection of a state of practice in swinging a golf driver, etc.
As for an image forming optical system used for such camera mounted on vehicle, it has been demanded that an image forming optical system has a large angle of view, concretely, diagonal angle of view is about 130 degrees or more, and high image quality, and further it is compact with a small number of sheets of lens, and can be produced by low cost, in order that a dead angle is made small as much as possible.
As the image forming optical system used for the camera for vehicle, a system in which a distortion generated is small has been demanded. In the optical system in which a large negative distortion is generated although it has a large angle of view as seen in an optical system using a fish-eye lens for example, there is a case that exact information on a circumference portion cannot be obtained since an image size of the circumference portion is very small compared with that of the central portion of the obtained image. Furthermore, there is a case that a shape of the image of an object that is actually observed with the naked eye may differ very much from that of an object image that is observed through a monitor since an image of a linear object is formed as a bent shape at the circumference portion.
Under such circumstances, examples of conventional image forming optical system in which an angle of view is comparatively large, and a distortion is corrected to some extent, and the number of sheets of lens is small have been shown in Publications of the Japanese unexamined patent application Toku Kai No. 2006-201674, Patent No. 2006-220691, Patent No. 2006-243092, and Patent No. 2006-301222.